Figure 8: a) Schematic Illustration of ion transport in WS cell wall, and electricity generation via water transportation through the micro/nanochannels. b) Potential profile in the EDL at the NS/water interface. c) Comparison of Voc on DI water and ethanol reservoirs. d) Effect of sealing on the output voltage. e) Illustration of the overall integrated underlined mechanisms of the high-performed WS-H+ device.
It is evident from the equation that the streaming potential Vs can be very high by decreasing the channel diameter and decreasing the channel length can give higher streaming current Is. The uneven diameter of the micro/nanochannels of the NS structures creates an elevated pressure difference (ΔP ) that facilitates achieving superior Vs and Is. However, very narrow channels introduce a high flow resistance which can be expressed by the Hagen–Poiseuille equation in Equation 5 . A very high resistance may cancel the driving force from the capillary pressure, preventing flow through the channel and resulting in a substantially lower voltage and current output. Therefore, suitable channel dimensions are essential for achieving the optimised higher power output. Fine tuning of the device performance and channel dimension is required.
The effect of concentrations, debeye length, and EDL overlap is explained in the supplementary section, Equation- S 1. Water-evaporation-driven electricity generation has typically been recognized entirely as streaming potential.[18] It is difficult to differentiate these two processes because of the absence of theoretical evidence and limited understanding of evaporation-induced capillary pressure and evaporation potentials. A very recent investigation focuses on illuminating the functions of evaporation potential and streaming potential in the generation of electricity.[19] Based on that point of view, in this study, we explored the influence of evaporation potential through the utilization of two different approaches. Initially, we substituted the water with ethanol, which has lower zeta potential. The device maintains the ability to produce an open circuit voltage of 254 mV, depicted in Figure 8 (c). A decrease from the initial 601 mV was observed with water as the medium. This implies that evaporation plays a partial role in the whole procedure, as clarified by the governing Equation- S 2 for the potential of evaporation produced by ethanol. Additionally, sealing the device with an encloser leads to a reduction in voltage output from 617 mV to 412 mV after 10 minutes, as it successfully minimized evaporation, portrayed inFigure 8 (d). These results emphasize the significant effect of both processes and signify that a substantial part of the voltage originates from the evaporation procedure.
This study aims to gather a full understanding of the reported events by inspecting the effects of different chemical treatments on ion transport and electricity generation. Whenever the WS-H+ samples were independently subjected to DI water and OH-solutions, the chemical reactions took place, which enhanced the output power. When H+ ions from the WS-H+approached to OH- ions of the solution, they interacted spontaneously as per the equation H+ + OH- = H2O. Therefore, continuous movement and migration of H+ ions occur in the acid-base process. As long as the top and bottom surface of the WS has concentration variations (different functional groups), the Voc and I sc show elevated values. The differential ion flow causes an elevation in both open circuit voltage and short circuit current, which is mostly due to chemical reactions and partly due to underlying physical phenomena, as illustrated inFigure 8 (e ). To evaluate the effect of acid-base reactions, an additional experiment was performed by soaking regular paper in acid and placing it in an alkaline reservoir. The results revealed a substantial voltage, as seen in Figure-S 7.

Power density analyses and scalability for practical applications

These WS-WEG devices can charge commercial capacitors of 47 µF, 470 µF, and 1000 µF, reaching the stable voltage of 615 mV without any secondary rectifier. Figure 9 (a) shows the required time to reach the peak voltage by these three capacitors is 15 s, 110 s which is very fast compared with the existing WEG devices [43], [60], [61].Figure 9 (b) shows the power density of the WS-WEG device with the varied resistance from 10 Ώ to 108 Ώ. The highest power density, 5.90 μW/cm2, of this WS-WEG device, was observed with the external load value of this WS-WEG device’s current-voltage (I-V) characteristics were assessed. The Figure 9 (c) exhibits a linear reduction in voltage while applying current. Despite the application of a negative current, it is shown as positive for the purpose of simplicity. The highest power density was consistently measured as 5.96 μW/cm². Scalability is essential for the practical use of any devices. Therefore, multiple devices were connected in series and parallel to justify their performance. Figure 9 (d) illustrates the Voc of 1.77 V while connecting four WS-WEG units in series and Figure 9 (e) shows the Voc of 3.18 V while connecting four WS-H+-WEG units in series. Therefore, two WS-H+-WEG units connected in series and two WS-H+-WEG units in parallel were able to power an LCD-screened calculator without the aid of any external boosters or rectifiers, as shown in Figure 9 (f) and S-V2. Figure 9 (g) depicts that these simple and innovative WS-WEG and WS-H+-WEG devices show outstanding performance compared with similar evaporation-driven WEG devices, which are based on conventional organic and inorganic materials.